mysqld_safe: use sh not bash

c902d5a changed this to bash.
This isn't appropriate as mysqld_safe is used by FreeBSD where there is no bash by default.

The original reason to use bash seems related to MDEV-3279 which
was a dash bug fixed in 2009 - https://git.kernel.org/pub/scm/utils/dash/dash.git/commit/?id=3800d4934391b144fd261a7957aea72ced7d47ea

Working around 9 year old fixed bugs shouldn't be done without considerations of other users. Debian/Ubuntu are using systemd now so the mysqld_safe implementation isn't as critical to them (right?).

Like 64094e1, some actual analysis was done in MDEV-14900 that could of been used instead of "validity of this has thus been vetted in production" (or more correctly, "as no-one has complained it must be right").